In recent years, wireless local area networks (LANs) of the IEEE802.11 standard have been widely used not only in companies and public spaces but also in ordinary homes with the spread of portable high-performance wireless stations such as laptop personal computers and smartphones. Wireless LANs of the IEEE802.11 standard include wireless LANs of the IEEE802.11b and IEEE802.11g standards using a 2.4 GHz band, and wireless LANs of the IEEE802.11a standard using a 5 GHz band.
In a wireless LAN of the IEEE802.11b or IEEE802.11g standard, 13 channels are prepared at intervals of 5 MHz between 2400 MHz and 2483.5 MHz. However, when a plurality of channels are used in the same place, if the channels are used without overlapping spectra so as to avoid interference, a maximum of three channels, or four channels in some cases, can be simultaneously used.
On the other hand, in the IEEE802.11a standard, 19 channels, including 8 channels and 11 channels that do not overlap between 5710 MHz and 5330 MHz and between 5490 MHz and 5170 MHz, respectively, have been defined in Japan. It is to be noted that, in the IEEE802.11a standard, a bandwidth per channel is fixed to 20 MHz (Non-Patent Document 1).
A maximum transmission rate of the wireless LAN is 11 M bits per second (bps) in the IEEE802.11b standard, and 54 Mbps in the IEEE802.11a standard and the IEEE802.11g standard. However, the transmission rate here is a transmission rate on a physical layer. Since transmission efficiency in a medium access control (MAC) layer is about 50 to 70%, an upper limit value of an actual throughput is about 5 Mbps in the IEEE802.11b standard and about 30 Mbps in the IEEE802.11a standard and the IEEE802.11g standard. Moreover, when a number of communication stations trying to transmit information increases, the transmission rate further decreases.
On the other hand, in wired LANs, provision of a high-speed line of 100 Mbps has become widespread with the spread of Fiber to the home (FTTH) using optical fibers, including a 100Base-T interface of the Ethernet (registered trademark), in individual homes, and further speedup of the transmission rate is required in the wireless LANs.
Accordingly, in the IEEE802.11n standard standardized in 2009, a channel bandwidth that was fixed to 20 MHz up to that time increased to 40 MHz at the maximum, and introduction of a spatial multiplexing transmission technology (a multiple input multiple output (MIMO) technology) was determined. When all functions defined in the IEEE802.11n standard are applied and transmission and reception are performed, a maximum communication speed of 600 Mbps can be realized in the physical layer.
Further, in the IEEE802.11ac in which a standardization specification is currently being investigated, an extension of the channel bandwidth to 80 MHz or a maximum of 160 MHz, introduction of a multi-user MIMO (MU-MIMO) transmission method to which space division multiple access (SDMA) is applied, and the like are being investigated. When all functions defined in the IEEE802.11ac standard are applied and transmission and reception are performed, a maximum communication speed of about 6.8 Gbps can be realized in the physical layer (for example, see Non-Patent Document 2).
Since wireless LANs of the IEEE802.11 standard are operated in frequency bands for which no license is required such as a 2.4 GHz band and a 5 GHz band, it is necessary for an access point that supports an IEEE802.11 wireless LAN (hereinafter referred to as an access point, which is indicated as AP in the drawings) to determine a frequency channel on which the wireless LAN access point is to be operated among frequency channels that can be handled by the wireless LAN access point itself when forming a wireless LAN cell (BSS: Basic Service Set). Further, in order to reduce interference, it is necessary to determine a transmission output value of the wireless LAN access point itself when another wireless LAN cell using the same or adjacent frequency channels exists within a range that electric waves of the wireless LAN access point itself reach.
Also, a cell is operated by describing set values of parameters used in the cell itself and other parameters that can be supported by the wireless LAN access point itself in a regularly transmitted beacon frame, a probe response frame in response to a probe request frame received from a wireless station, or the like, and transmitting the frame on a frequency channel determined to be operated to notify associated wireless stations and neighboring communication stations of such values and parameters.
For example, the set values of the parameters used in the cell itself include a parameter value regarding acquisition of access right and a parameter value such as QoS (Quality of Services). Further, the other parameters that can be supported by the wireless LAN access point itself include a bandwidth used for transmission of frames, and a data rate set regarding a basic data rate (BSS: Basic Rate Set) used for transmission of control frames and a data rate at which transmission and reception of data can be performed, or the like.
Examples of a method for selecting and setting a frequency channel, a transmission power value, and other parameters in the wireless LAN access point include (1) a method in which a default parameter value set by a manufacturer of the wireless LAN access point is used as it is, (2) a method in which a value manually set by a user who operates the wireless LAN access point is used, (3) a method for, when each wireless LAN access point starts up, autonomously selecting and setting a parameter value based on wireless environment information detected in each wireless LAN access point itself, and (4) a method for performing setting using a parameter value determined by a centralized control server such as an access point controller.
When a bandwidth per channel is widened to 40 MHz, 80 MHz, or 160 MHz as described above, the number of channels that can be used simultaneously at the same place in a 5 GHz band decreases to 9 channels, 4 channels, or 2 channels, respectively. That is, the number of usable channels decreases as the bandwidth per channel increases.
Further, because the number of prepared channels that can be simultaneously used at the same place is 3 in the wireless LAN of the 2.4 GHz band and 2, 4, 9, or 19 in the wireless LAN of the 5 GHz band, it is necessary for an access point (AP) to select a channel to be used in its own cell (BSS: Basic Service Set) when the wireless LAN is actually introduced.
In an environment in which the number of BSSs is greater than the number of available channels, a plurality of BSSs use the same channel (OBSS: Overlapping BSS). In the wireless LAN, autonomous distribution access control in which data transmission is performed only when a channel is empty using carrier sense multiple access with collision avoidance (CSMA/CA) is used.
Specifically, a communication station in which a transmission request is generated first monitors a status of a wireless medium during a predetermined sensing period (DIFS: Distributed Inter-Frame Space), and performs random back-off if there is no transmission signal from other communication stations during this period. The communication station continues to monitor the wireless medium during a random back-off period, and obtains a channel use right if there is no transmission signal from the other communication stations during this period. The communication station obtaining the channel use right can transmit data to the other communication stations in the same BSS and receive data from the communication stations. Since such control is performed, throughput to be obtained is degraded if there are many competing communication cells or communication stations. Therefore, it is important to monitor a surrounding environment and select an appropriate channel.
Since a channel selection method in an access point is not defined in the IEEE802.11 standard, each vendor uses its unique channel selection method, but a most general channel selection method is a method for selecting a channel with minimum interference power. The access point detects statuses of all channels for a constant period (executes scanning), selects a channel with minimum interference power, and performs transmission and reception of data with communication stations associated therewith on the selected channel. It is to be noted that the interference power is a level of a signal received from a neighboring BSS or another system.
Further, a procedure of changing the channel when a wireless situation around the BSS is changed is defined in the IEEE802.11 standard, but reselection of a once selected channel is not basically performed except for forced transition due to, for example, radar detection. That is, in current wireless LANs, channel optimization in accordance with a change in the wireless situation is not performed.